There are a lot of interesting applications that use similar technology. A lot of labs that work towards quantum computing also publish papers with these applications.
Here are some:
All-optical computation. Personally, I think this has more potential than quantum computing, as it has already been shown to be useful for quickly processing neural networks (and other algorithms involving matrix multiplication and nonlinear functions). These on-chip systems are made in the same labs (and same people) as measurement-based linear quantum computing. Designing systems capable of operating faster than semi-conductor clock speeds, lowering the minimum power-per operation using light, and increasing parallelization will probably get us very far without needing to change algorithmic architectures.
Quantum simulation. Richard Feynman's original dream of "quantum computers" are now what are referred to as "quantum analog simulators." Nature acts like nature. It can be hard to compute analytically or digitally how a Hydrogen atom behaves, but using a system with a similar Hamiltonian can "do the math for you." Optical lattices (which are sometimes used for quantum computing of ions) can be used for these quantum simulators. It is very difficult to do calculations of molecules using fundamental physics and chemistry is full of heuristics to deal with these difficulties.
Quantum state reconstruction. A usually unmentioned open problem in quantum information and computing is how to reconstruct high qbit entangled states. Even if quantum computing doesn't work out, advances made in these open questions might be helpful in the future (for, for instance, key distribution protocols and information theory).
Quantum Communication. Quantum Key distribution is probably the only working practical application created so far from quantum information. It allows information to be transferred safely without the possibility of eavesdroppers. High-fidelity photon gate operations (created for quantum computers) could allow for efficient quantum repeaters, which could extend the maximum distance that can be traveled.
Extra Fun Things. Personally, I think the most interesting thing is answering if the brain is a quantum computer. The possiblity of the brain being a quantum computer has been eye-rolled by many physicists for the last decade, dismissing the high temperatures the brain to destroy coherence, but highly reputable (and commendable) physicists have recently challenged this notion. One discussing how nuclear spins could be the mediator of quantum information, another discussing how experiments could be carried to investigate if axons are operating as waveguides.
